The present invention relates to apparatus for softening water; and particularly to systems for controlling the regeneration of the resin of the water softener.
It is not uncommon for water that is drawn from a well to be considered "hard" in that it contains di-positive and sometimes tri-positive ions which have leached from mineral deposits in the earth. Such ion form insoluble salts with common detergents and soaps producing precipitates that require an increased quantity of detergent or soap for cleaning purposes. When hard water is used in boilers evaporation results in the precipitation of the insoluble residues which tend to accumulate as scale.
It is common practice to install a water softener in the plumbing system of a building that is supplied with hard water. The most common type of water softener is an ion exchange resin-type softener having a tank which holds a bed of resin through which the hard water is passed to remove undesirable minerals and other impurities. Initially binding sites in the resin bed contains positive ions, commonly unipositive sodium or potassium ions. As hard water enters the resin, competition for the binding sites occurs. The di-positive and tri-positive ions in the hard water are favored due to their higher charge densities and displace the unipositive ions. Two or three unipositive ions are displaced for each di-positive or tri-positive ion, respectively.
The capacity of the rein bed to absorb minerals and impurities is finite and eventually ceases to soften the water when a large percentage of the sites are occupied by the di-positive and tri-positive ions. When this occurs, it becomes necessary to recharge or regenerate the resin bed by flushing it with a regenerant, typically a solution of sodium chloride or potassium chloride. The concentration of unipositive ions in the regenerant is sufficiently high to offset the unfavorable electrostatic competition and the binding sites are recovered by unipositive ions. The interval of time between regeneration periods during which water softening takes place is referred to as a "service cycle."
Regeneration of early types of water softeners was affected manually only after it was discovered that the treatment capacity of the resin bed has been exceeded and the water flowing therethrough is not longer soft. In an effort to eliminate the need for manual regeneration, water softener control systems were developed utilizing a mechanical clock which initiated water softener regeneration on a periodic basis. The frequency of such regeneration was set in accordance to the known capacity of the resin bed and the anticipated daily usage of soft water. While mechanical clock-type water softener controllers have alleviated the need for manually regenerating the resin bed, such controllers are subject to the disadvantage that by regenerating at fixed intervals, regeneration may occur too often or too late depending upon water usage. Regenerating the water softener resin bed when sufficient capacity to treat water still exists is wasteful of the regenerant and the water used in regeneration. Conversely, failure to regenerate the water softener after the resin bed capacity has diminished to a point below that required to treat hard water results in hard water leaving the water softener.
In an effort to better regulate the frequency of water softener resin bed regeneration, demand-type water softener control units have been developed which determine the remaining capacity of the water softener resin bed to soften water. One type of such an improved controller is disclosed in U.S. Pat. No. 4,426,294 in which a flow meter measures the volume of water being treated and regenerates the resin bed when a specified volume of water has flowed through the softener since the previous regeneration. While this type of system is adequate in many installations, municipal systems alternately may draw water from several wells which contain water having different degrees of hardness. In this case, the exhaustion of the resin bed is not a direct function of the volume of water which has been treated since the previous regeneration.
Other types of control systems were developed which attempted to measure the exhaustion of the resin bed directly. For example, U.S. Pat. No. 4,320,010 placed electrodes in the resin bed to detect an electrical voltage generated by the resin bed. This voltage changed when the resin bed became exhausted, thus providing a mechanism by which the controller could determine when regeneration was required. Another control technique measured the conductivity between two pairs of electrodes placed at different levels in the resin bed. The difference in conductivity between the two locations in the resin bed was used to determine when exhaustion of the water softener had occurred. A system of this type is disclosed in U.S. Pat. No. 4,299,698. A similar technique is described in U.S. Pat. No. 3,618,769 in which the ratio of the conductivity of two locations in the resin bed is used to determined when regeneration should take place.